1. Field of the Invention
This invention relates to semiconductor manufacture and more particularly to a device and method for implanting a substantially uniform dosage of ions across a semiconductor wafer.
2. Background of the Relevant Art
Ion implantation devices, or ion implanters, are well known. A primary function of an ion implanter is to provide an ion beam with sufficient energy to penetrate into a solid surface. Ion implanters are commonly used in the semiconductor industry to introduce impurities or dopants into the solid substrate material. The depth in which ions are introduced into the substrate increases as acceleration voltage of the implanter increases. Moreover, the total number of ions injected is proportional to the beam current and implant time of the implanter.
Due to the growing popularity of high density VLSI circuits, medium to low current implanters have become a mainstay in integrated circuit manufacture. It is important when implanting impurities that the impurities be placed at a fairly shallow distance into the substrate. As such, the impurities form a shallow junction necessary for enhanced performance of high density circuits. Medium to low current implanters are generally characterized as implanters having beam currents generally less than 10 .mu.A. A typical medium current implanter 10 is shown in FIG. 1. Implanter 10 generally includes an ion source 12, a target area 14, and an ion beam 16 conditioned between source 12 and target 14. Placed within the path of ion beam 16 are numerous conditioning elements. In particular, a mass analyzer 18 is used to filter or select various ion species based on the charge-to-mass ratio of the extracted ions. Analyzer 18 is generally sensitive enough to discriminate against adjacent mass numbers. The ions are then given a final acceleration from a resolving aperture 20 via an acceleration tube 22. The accelerated ions produced at the output of tube 22 are then focused to a fairly close or "tight" beam width via lens 24. The focused beam can them be deflected by x- and y-deflector plates 26 and 28, respectively. X- and y-deflector plates receive modulated voltage so as to scan the focused ion beam 16 in the x and y directions, respectively, across a wafer placed within target area 14.
Ion beam 16 is focused and deflected by plates 26 and 28 in the same manner as electron beams are deflected in electron microscopy. Ion beam lens 24 and plates 26 and 28 function by electrostatically shaping an electron or magnetic field thereby causing a resulting effect upon the charged ion beam. Beam 16 is focused at a specific beam width of ions impinging across select areas of a semiconductor wafer. The methodology by which beam 16 is electrostatically controlled in a specific scan pattern across a wafer is herein described below.
Referring to FIG. 2, lens 24 can be made from a pair of hollow-tube, electrostatically charged cylinders 30a and 30b. Cylinders 30a and 30b have electrical energy applied to them to focus or converge an ion source of beams 16 along a central axis 32. Unfortunately, depending upon the relative atomic mass of the ion species being accelerated, varying amounts of convergence is achieved. For example, lighter boron ions may be more easily converged along axis 32 than heavier phosphorous ions, provided the amount of charge on cylinders 30a and 30b remains fixed. However, lighter boron ions have a greater tendency to diverge downstream of lens 24. The lighter boron ions are more susceptible to extraneous electric or magnetic fields which can pull the ions from a tightly focused beam width.
Referring to FIG. 3, the effects of extraneous electrical and magnetic fields upon the beam width, between lens 24 and target 14, is illustrated as a function of atomic mass of the ion species. A heavy atomic species 34 often maintains a tighter beam width, BW.sub.EFF, than a light atomic species 36. As such, beam size (i.e., beam width or beam diameter) is a function of relative atomic mass of the species being accelerated. During semiconductor manufacture, the operator must be cognizant of the atomic species in order to ensure that the resulting beam width scans in a uniform fashion across the entire wafer surface. If the beam width is more narrow, as is often the case in heavier atomic species, then the scanned pattern must be changed accordingly. Otherwise, there may be "gaps" between adjacent beam width scan lines. The gaps can lead to areas of underdosage upon the wafer.
FIG. 4 illustrates the "gaping" problem which may arise if the ion implanter is not adjusted to compensate for smaller beam widths, BW.sub.EFF, attributed to larger atomic mass species. Gap 40 is shown to occur on wafer 42, between adjacent scan lines 44a and 44b. Gap area 40 receives insufficient amounts of ion implant thereby causing, for example, relative increase in thresholds of active devices patterned within area 40.
As shown in FIG. 4, ion beam 16 is scanned across wafer 42 in both the x and y directions. Scanning occurs by modulating voltage on both the x and y deflector plates 26 and 28, shown in FIG. 1, so as to move the ion beam in a positive x and y direction followed by a negative x and y direction, thereby completing an entire scan cycle. To further illustrate, a "scan cycle" is the scanning of an ion beam completely across the wafer and then rescanning the ion beam back across the wafer. The scan cycle includes movement of the beam in a specific pattern, in accordance with the signal frequency placed upon the deflectors. The pattern maintains or repeats itself throughout the entire scan cycle causing a property often referred to as "retrace".
Retrace involves movement of the ion beam during one half a scan cycle across wafer 42 and then moving the ion beam in the very same pattern back across wafer 42. The retrace pattern involved in the second half of the scan cycle is the same ion beam pattern placed upon wafer 42 during the first half of the scan cycle. Thus, if there is any gap 40 between beam widths, the retrace pattern will not "fill" the gap and thus cannot prevent non-uniform dopant placed across wafer 42. Knowing the importance of uniform doping, it is therefore advantageous to provide a scan pattern which would ensure there would be no gaps upon wafer 42 during times in which ion source 12 undergoes a change in source material having a heavier atomic mass. It is further advantageous that the shape of the beam be irrelevant to the uniformity of the dopant placed across the wafer.